The coating of electrically conductive substrates by an electrodeposition process, also called an electrocoating process, is a well-known and important industrial process. Electrodeposition of primers on metal automotive substrates is widely used in the automotive industry. In this process, a conductive article, such as an autobody or an auto part, is immersed in a bath of an aqueous emulsion of film forming polymer and the article acts as an electrode in the electrodeposition process. An electric current is passed between the article and a counter-electrode in electrical contact with the coating composition until a coating is deposited on the article. In a cathodic electrocoating process, the article to be coated is the cathode and the counter-electrode is the anode. In an anodic electrocoating process, the article to be coated is the anode and the counter-electrode is the cathode.
Film forming resin compositions used in the bath of a typical cathodic electrodeposition process also are well known in the art and have been in use since the 1970's. These resins typically are made from polyepoxide resins that have been chain extended with an amine compound(s). The epoxy amine adduct is then neutralized with an acid compound to form a water soluble or water dispersible resin. These resins are blended with a crosslinking agent, usually a polyisocyanate, and dispersed in water to form a water emulsion which is usually referred to as a principal emulsion.
The principal emulsion is combined with a pigment paste, coalescent solvents, water, and other additives such as film build additives, pinhole additives and anti-crater agents to form the electrocoating bath. The electrocoating bath is placed in an insulated tank containing the anode. The article to be coated is the cathode and is passed through the tank containing the electrodeposition bath. The thickness of the coating that is deposited on the article being electrocoated is a function of the bath characteristics, the electrical operating characteristics of the tank, the immersion time, and the like.
Anodic electrocoat compositions, while known, account for only a small percentage of the electrocoating industry. The first automotive electrocoat systems were anodic but were plagued by inadequate corrosion resistance, staining of the cured film and sensitivity to the substrate. The anodic electrocoat compositions were largely replaced in the mid-1970's by cathodic electrocoatings.
Compared with cathodic electrodeposition, articles coated with known anodic electrodeposition compositions typically have poor corrosion resistance, poor chip resistance, and poor flexibility. While cathodic coatings are more widely used than anodic electrodeposition coatings, cathodic electrodeposition coatings still suffer from problems, such as having limited UV stability, poor resistance to deformation, poor resistance to chipping, i.e., resistance to impacts from stones or other materials that may contact a substrate coated with at least a layer of electrodeposited coating.
There remains a need for electrodeposition coatings that have improved UV resistance, better resistance to deformation and have improved chip resistance.